In Koppen's climate classification system, the primary climate group letters denote broad climate categories: A designates tropical (rainy) climates, B designates dry (arid and semi-arid) climates, C designates mild, mesothermal, temperate humid climates, D designates snowy, microthermal continental (snow-forest) climates, and E designates polar climates; correctly matching each letter to its designated climate category is essential, since B (dry climates) is frequently and incorrectly confused with E (polar climates) despite representing an entirely different, moisture-based rather than temperature-based, climate criterion.